Process for absorbing toxic gas

ABSTRACT

A process for absorbing noxious components such as SO x , NO x , and HX from a gas such as a flue gas at temperature below 250° C. is improved by contacting the gas with a conditioned solid acceptor that contains divalent metal oxide, hydroxide or both coated onto exfoliated vermiculite and/or expanded perlite. Before use (to enhance its absorption of noxious components) conditioning can be done by heating the hydrated acceptor at temperature of at least about 400° C. for a short time.

RELATED APPLICATIONS

This application is a division of applicant's copending application Ser.No. 10,449 filed on Feb. 3, 1987, now U.S. Pat. No. 4,721,582 of Jan.26, 1988, entitled Toxic Gas Absorbent and Process for Making Same whichitself was a continuation-in-part of applicant's now abandonedapplication Ser. No. 838,354 filed on Mar. 11, 1986, entitled Materialand Process for Gas Desulfurization. The teachings of thesespecifications are incorporated herein expressly by reference.

TECHNICAL FIELD

This invention relates to solid sorbent materials ("acceptors") for usein removing ("accepting") sulfur oxides, nitrogen oxides (NO_(x)),hydrogen halides and/or other noxious components (e.g. hydrogen sulfide,carbonyl sulfide, sulfur trioxide, etc.) from a gas, particularly awaste gas stream, and to improvements in process for making, using andregenerating the acceptors. Of these contaminants the ones speciallytargetted here are SO₂ and NO_(x) (typified by NO₂) as they as the mostcommon and usually most difficult to remove virtually completely amongstthose above specified.

BACKGROUND ART

Flue or vent gases from the combustion of fuels such as coal or residualoil, the pyrolysis of solid waste, and the venting of chemical processeshas been a matter of increasing concern for many years. Components suchas sulfur dioxide and nitrogen oxides (NO_(x)) are accused of fosteringacid rain, having toxic effect on human, lower animal and plant life,and being generally dangerous or obnoxious even in quite lowconcentrations.

As a result, many approaches have been proposed to cleanse the wastegases of their toxic components. Scrubbing with a compound in slurryform with water has been by far the most popular approach. Wet scrubbingsystems, although effective in removing toxic gases, have been veryexpensive and highly corrosive and have often simply converted anair-pollution problem into a sludge-disposal or water-pollution problem.Even with wet scrubbing processes involving reactants that areregenerable, thus producing no waste sludge, problems have occurred. Inthe magnesium oxide wet scrubbing process, for example, major technicaldifficulties have been encountered in handling the corrosive liquidslurries, and large energy costs have been incurred in drying theslurries and in regenerating the magnesium oxide.

In recent years several so-called dry sorbent processes have beenproposed. In these processes particulate reactant is brought into directcontact with the undesirable components of a waste gas stream where itreacts with those components in a dry or semi-dry state. Principaladvantages of these processes have been their simplicity, decreasedmechanical problems, and generally lower costs. A major disadvantage hasbeen their low cleansing efficiencies; at best generally only 50 to 60percent of the toxic components are removed.

Various approaches have been examined in an attempt to increase thereactivity and cleansing efficiency of particulate reactants. One suchapproach involves the use of silicate particles having a huge surfacearea, e.g., asbestos, exfoliated vermiculite, or expanded (popped)perlite.

Thus, U.S. Pat. No. 4,061,476 teaches that expanded perlite optionallyis added to a solid absorbent. Pulverulent amorphous silicon dioxide,charcoal, carbon dust, lime hydrate, bog iron ore, pulverulent ironoxide, and dolomite dust are taught as being useful solids calledvariously "absorption" or "sorption" agents or "absorbents". Poppedperlite is stated to have good physical absorption properties and issuitable for supplying water to the reaction; alternatively the gas inprocess can be humidified.

U.S. Pat. No. 4,201,751 to the same inventors teaches use of exfoliatedvermiculite and expanded perlite, suitably moistened with water andcaustic soda, as a carrier for a sprayed-on aqueous suspension ofabsorption agent. Such agent is rich in ferric oxide-containing steelmill dust; higher alkalinity is imparted to the suspension by dispersingcalcium hydroxide and/or calcium oxide therein; the further mixing in ofbog iron ore and/or a heavy metal salt such as one of lead or copper isstated to improve the sorption of various acidic gases. Lime-alkalizedsteel mill dust on an expanded perlite carrier is exemplified fortreating contaminated gas. When the absorption agent becomes exhaustedby contact with SO₂, it can be roasted to drive off SO₂ withpreservation of the perlite. Perlite does not tend to sinter until above980° C.

An earlier patent, U.S. Pat. No. 3,882,221, appears to prefer aqueouscaustic soda or potash solutions coated onto exfoliated vermiculite orexpanded perlite for gas cleaning purposes. However, this patentexemplifies wetting one or the other of these two expanded minerals withan aqueous dispersion of calcium oxide and of calcium hydroxide to coatsuch carrier, then reacting the coatings with chlorine and carbondioxide gases, respectively. Details of the preparation and operatingefficiency of these chalky coatings are not evident from the text, andtheir effectiveness for removing SO₂ and NO_(x) only can be speculatedon. The reaction products obtained are stated as being capable of beingremoved from the expanded mineral with water or other unnamed suitablesolvent.

It has now been discovered that exfoliated or expanded minerals, such asvermiculite or perlite, freshly coated with a hydrated divalent metaloxide, can be conditioned in a simple manner to produce an improvedacceptor for cleansing waste gases at temperatures below 250° C.Complete and efficient removal of mixed SO₂ and NO₂ has been obtainedconsistently when using gases with contaminant concentrationsrepresentative of those in waste gases, such as a flue gas from thecombustion of coal. Furthermore, an improved coating method has beendevised for obtaining very high, waste-free loading of MgO and CaO onthe exfoliated or expanded mineral for extended sorption service. Theconditioning method has been found to result in an interaction betweenthe MgO or CaO and the mineral carrier that results in a strong bondbetween the phases. Also, the conditioning has been found to increasemarkedly the strength of the carrier, resulting in significantly lessdegradation in handling. Additionally, the magnesium oxide embodiment ofthe improved acceptor has been found, surprisingly, to be repeatedlyregenerable without noticeable degradation at a much lower temperaturethan related prior art acceptors, and that regeneration can be practicedefficiently in a manner that allows the simultaneous removal andseparate collection or destruction of SO₂ and NO_(x) when these gaseshave been sorbed together.

BROAD STATEMENT OF THE INVENTION

I have found, surprisingly, that a solid acceptor for removal of noxiouscomponents from gas at temperature below 250° C., which acceptor isparticulate exfoliated or expanded mineral support coated with hydrateddivalent metal oxide, can be demonstrably improved for the acceptingpurpose by a mild, short term, heat treatment of such hydrousassociation of oxide and support before its use. The heating is for atleast a few minutes at a temperature substantially above that of normaluse, i.e., at a hypernormal temperature such as 450°-650° C. For brevityI refer to the operation as one of "conditioning", and the improvedacceptor as one that has been "conditioned" for the accepting purpose.Obtaining very high useful loadings of metal oxide such as MgO and CaOon the support involves saturating or nearly saturating the poroussupport with water, and then applying the oxide in an ostensibly dry,pulverulent condition to the wet support while the latter is beingagitated. Thus loaded, the material can be conditioned by the mild,short term heat treatment.

Regeneration of the improved magnesia acceptor that has become ladenwith contaminant gas can be accomplished by simply heating the acceptorto conditioning temperature and maintaining it there until the sorbedgases (or their gasiform derivatives) are substantially expelled fromthe acceptor. With this procedure, SO₂, NO_(x) and HCl can be desorbedfrom the acceptor. This relatively low temperature thermal regenerationcan be performed in the presence of a gasiform reducing agent such asmethane, if desired.

BEST MODE FOR CARRYING OUT THE INVENTION

Advantageous metal oxides for the instant sorption purpose are MgO, CaO,ZnO and CuO, preferably MgO and CaO, and most preferably MgO. Theseoxides need not be of high purity. Use of technical or agriculturalgrades and even much lower grades is satisfactory. Carbonatecontamination can be expected in some magnesias and calcium oxides,particularly if they have been stored for long times with no specialprecautions. For the present purposes, magnesium carbonate presents nodifficulties because it decomposes to MgO at the modest conditioningtemperatures. Calcium and dolomitic carbonates, on the other hand, donot decompose appreciably if at all at the conditioning temperatures.While it is possible to substitute here calcium hydroxide for some orall of the CaO and magnesium hydroxide for some or all of the MgO, thiscould result in reduced sorption capacity per unit weight of thedivalent metal equivalent applied, hence is less favored than use of theoxides.

Various methods of contacting the contaminated gas with the acceptor canbe practiced. The preferred way involves passing such gas through one ormore static or agitated beds of the improved acceptor particles, thendirecting the cleansed gas to a stack. The gas can be passed upwardly,downwardly, or in a cross flow direction with respect to the bed. Thebed can be vertical, horizontal, raked or rabbled, and it can be static,slowly moving downwardly or horizontally, bubbling or in fluid motion.

Alternatively, acceptor particles can be injected into a waste gasflowing through a duct and turbulently entrained with the gas andfinally collected as a bed on a screen or fabric filter. In suchinstances, however, it is probably desirable to maintain a modest sizedbed of particles (at least several inches deep) on the screen or filterto bring about longer and probably more thorough contact between theabsorbent and the gas.

The use of moving beds or a dry injection system has the advantage ofpermitting separated, laden acceptor to be continuously drawn off,regenerated, and recycled for reuse. This can be particularly practicalwhen using the present MgO acceptor embodiment where regeneration can bedone at a low temperature. Efficient fixed bed practice often involvesthe use of two or more beds, each used and regenerated in an appropriatesequence.

Characteristically, as a bed of the instant improved acceptor cleansesthe incoming gas of SO₂, NO_(x) and/or HCl, an advancing "front"(interface) between the virtually spent acceptor and the more activeacceptor travels through the bed in the direction of gas flow. Thisoccurrence is especially and advantageously evident when a toxic gassuch as SO₂ is being sorbed by the present MgO acceptor embodiment withperlite as the mineral carrier. In this case, the normally snow whiteparticles turn bright yellow as the bed becomes saturated and the frontmoves through the bed.

In the case of a fixed bed, when or just before some preselected valuesuch as 50 percent of the contaminating gases are noted as passingthrough and out from the filter bed without reacting, the gas flow canbe switched to a fresher bed. In the case of moving beds, the beds canbe continuously or intermittantly augmented with active acceptormaterial, usually with some withdrawal of spent material. Advantageouslythe gas and/or the acceptor is humidified for the sorption operationunless the gas already holds considerable water.

Conditioning of the instant, freshly made acceptor is relatively simple.It is accomplished by heating the acceptor in air (or other gas notappreciably reactive with the acceptor) at temperature above 400° C.,and preferably at 450° to 550° C. for at least a few minutes,advantageously 15 or more minutes, and preferably for 20 minutes. Theheating can be done in a furnace, kiln, or the like, or in situ at thegas cleansing location. Conditioning for 1/2-1 hour or even longer doesnot appear to harm the acceptor; it just costs more.

The solid acceptor of the present invention preferably is prepared byfirst weighing out a given quantity of mineral carrier, and then mixingit with a weighed quantity of water wherein the weight ratio of water todry mineral carrier does not exceed about 4:1 and no free or standingwater exists after mixing. A weighed quantity of dry divalent metaloxide, in powder form (preferably passing through 28 mesh Tyler Standardsieve) is then sifted into the wet carrier as the latter is stirredslowly and continuously. The quantity of oxide blended into the mixturecan vary, but should not exceed an amount that is equivalent to a weightratio of about 60:40 of dry oxide to dry carrier. If a weight ratiohigher than about 60:40 is used, generally a significant number ofunattached oxide particles tend to exist in the final acceptor material.

Mixing can be performed manually or mechanically by using any one of anumber of commercially available mixers or slow blenders. Manual ormechanical mixing with the aforementioned procedures has been found toresult in an oxide phase uniformly distributed on the mineral carrier,in few or no free (unattached) oxide particles, and in no significantbonding together of coated mineral carrier particles; they remaindiscrete. The moist acceptor particles advantageously are then permittedto air dry for a period of 30 minutes or more, after which time they arerelatively free flowing and are ready for conditioning.

Before conditioning, the acceptor will usually contain a significantamount of water. The porous acceptor (exfoliated vermiculite or expandedperlite) acts as a very effective reservior for water, allowing smallquantities of water to escape and to react with oxide particles attachedto the acceptor forming hydroxides. The water used can, if desired, bymade slightly alkaline with the addition of caustic soda, ammonia or thelike or slightly acid with the addition of an acid such as sulfuric acidbefore mixing. However, such alkalizing or acidifying steps aregenerally not needed.

During drying and conditioning at 400° to 650° C., particularly usefulcracks, fissures, and pores in the oxide coating are formed and areopened up as a result of the rapidly escaping water and the hightemperatures. Conceivably this occurrence may be partially responsiblefor the markedly improved effectiveness of conditioned absorbent overabsorbent with no conditioning.

After the acceptor material is employed to remove SO₂, NO_(x), etc. froma waste gas stream and approaches becoming saturated with suchcontaminants, the contaminants can be expelled from the acceptor byheating it. The instant magnesia acceptor embodiment actually can beregenerated repeatedly at conditioning temperature. An advantageousregeneration temperature range for the magnesia acceptor is 450° to 650°C.; the required time for complete regeneration decreases with anincrease in temperature. Higher temperatures, therefore, favor shorterregeneration times. Higher temperatures, however, often mean higherenergy costs, and heating the spent acceptor substantially above 650°C., e.g. 900° C. or above, can result in noticeable structuraldegradation of the acceptor. A satisfactory and economical and thereforepreferred regeneration temperature and time for the magnesium acceptoris 600° C. and 20 minutes, respectively. During regeneration, after aninitial emission of moisture which principally comes off at temperaturesslightly above 100° C., SO₂, NO_(x), HCl or other gases captured duringabsorption are released. Recovered SO₂ can be purified and liquified toprovide a valuable, marketable by-product. It is used in sewagetreatment, sulfuric acid manufacture, and brewing and has many otherapplications.

The rate and extent of regeneration can be improved by providing thespent sorbent with a reducing environment during heating. Smallquantities of methane, CO or elemental hydrogen added to the atmosphereof the regeneration unit provide such an environment. The use of methaneis particularly attractive when NO₂ is present in the gas beingrecovered from the acceptor. Methane will react with the NO₂, resultingin CO₂, H₂ O and nitrogen, all relatively innocuous species that can bedealt with readily in recovery processing.

The following examples summarize the preparation of and various testsperformed in connection with the improved acceptor, but should not beconstrued as limiting the invention. In this specification all parts areweight parts, all gas compositions are by volume, and all temperaturesare in degrees Celsius unless otherwise expressly noted.

For the following tests the exfoliated vermiculite support particles(approximately 0.1-0.25" in their largest dimension) were coated asfollows: 200 parts of water were soaked up by 60 parts of suchvermiculite at room temperature, about 22° C. This was short ofsaturation. The wet mixture then was stirred while 60 parts of drypowdered metal oxide was fed gradually onto the support phase and pickedup by it to make a mass of discrete, pourable but unconditionedparticles.

The coated expanded perlite was made as follows: 74 parts of water weresoaked up by 45 parts of such perlite (of approximately the same size asthe vermiculite above) at room temperature. This was short ofsaturation. The wet mixture then was stirred while 60 parts of drypowdered metal oxide was fed gradually onto the support phase and pickedup by it to make a mass of discrete, pourable but unconditionedparticles.

Of the metal oxides used, the calcium oxide, magnesium oxide, anddolomitic lime (CaMgO₂) were of technical grade, and the copper oxidewas obtained from waste cupric hydroxide sludge by heating and grinding.All oxides were screened through a 28-mesh Tyler Standard sieve beforebeing applied to the carrier materials. The conditioned acceptors weremoistened with several drops of water before use, but the unconditioned(air dried) acceptors were not so moistened.

The test gases of known compositions were prepared by a commercialvendor. They were saturated with water by bubbling them through boilingwater at atmospheric pressure, then conducted downflow through smallfixed beds of the test acceptor. Gas rates were reckoned at roomtemperature and one atmosphere total pressure. No significant pressuredrop was noted across filter beds during runs involving the improvedacceptors. From the examples it can be seen that the terms "activated","potentiated" or "capacitated" could be used as an alternative to theterm "conditioned".

EXAMPLE 1

Two 20-gram quantities of acceptor (MgO coated onto exfoliatedvermiculite) were conditioned by heating each at 550° C. for 30 minutes,then cooled and moistened. Two other like batches simply were air driedfor 2 hours. All were tested with a gas stream flowing at two liters perminute and containing 931 ppm of NO₂, 210 ppm of NO, and the balancenitrogen. The NO₂ removal even after two hours was extremely high forthe conditioned acceptors and essentially zero for the unconditionedones. About half of the NO was removed by the conditioned acceptorswhile significantly less NO was removed by the unconditioned ones.

The tests are summarized below; they demonstrate that conditioningdefinitely improves sorptive performance of the acceptor.

                  TABLE I                                                         ______________________________________                                        NO.sub.2 Removal Efficiency, Percent                                                        After      After   After                                        Acceptor      2 min.     10 min. 120 min.                                     ______________________________________                                        Unconditioned                                                                             #1     67.8      46.3  0                                          Unconditioned                                                                             #2     66.7      35.6  0                                          Conditioned #1    100.0      97.3  87.1                                       Conditioned #2    100.0      97.9  87.1                                       ______________________________________                                    

EXAMPLE 2

Unconditioned acceptors were made from: (a) exfoliated vermiculite andmagnesium oxide; (b) expanded perlite and magnesium oxide; and (c)exfoliated vermiculite and calcined dolomite. After being air-dried forthree hours these acceptors were separated into duplicate batches. Onebatch of each material was exposed directly to a five liter per minutesulfur dioxide-containing flue gas stream for 80 minutes. Itscorresponding second batch, after being given a conditioning treatmentconsisting of heating to 450° C. for 30 minutes, cooling, and moisteningwith a few drops of water, was likewise exposed to a five liter perminute sulfur dioxide-containing flue gas. The composition of the fluegas was: 20.18% carbon dioxide; 4.04% oxygen; 0.33% sulfur dioxide; andbalance nitrogen. The results of these runs, shown in Table II, indicatethe improved effectiveness of conditioned material over non-conditionedmaterial for removing sulfur dioxide from a sulfur dioxide-containinggas.

                  TABLE II                                                        ______________________________________                                        SO.sub.2 Removal Efficiency, Percent                                                          After     After   After                                       Acceptor        2 min.    50 min. 80 min.                                     ______________________________________                                        Vermiculite + MgO                                                                             100       0       0                                           Not Conditioned                                                               Vermiculite + MgO                                                                             100       86.4     57.6                                       Conditioned                                                                   Perlite + MgO     99.7    0       0                                           Not Conditioned                                                               Perlite + MgO   100       99.4     75.8                                       Conditioned                                                                   Vermiculite + CaMgO.sub.2                                                                     100       63.6    0                                           Not Conditioned                                                               Vermiculite + CaMgO.sub.2                                                                       99.8    69.7    0                                           Conditioned                                                                   ______________________________________                                    

EXAMPLE 3

Small (5-8 gram) beds of magnesia-based acceptors were conditioned,cooled, moistened with a few drops of water, and used to sorb aparticular contaminant from a stream of nitrogen flowing at 2 liters perminute. When they were practically saturated with contaminant, the bedswere regenerated by heating to drive off the contaminant, then recooled,remoistened and reused to sorb more of the contaminant. In two of thecases below three successive sorption cycles were run with first andsecond regenerations after the first and second sorption cycles.

In the case of HCl sorption the heating steps for conditioning and forregeneration were at 550° C. for 20 minutes; in the cases of SO₂ and NO₂sorption, such heatings were at 600° C. for 20 minutes. The results ofthese gas filtering experiments are tabulated below. The consistancy inthe time of breakthrough for 50% contaminant concentration of the gasindicates an excellent low-temperature regenerability that issubstantially non-destructive to the acceptor.

                  TABLE III                                                       ______________________________________                                                           Initial  Initial                                                              Conc. to Conc. out                                                                             Time at                                             Contam-  Bed, ppm Bed, ppm                                                                              50%                                       Filter    inate    (Volume  (Volume Breakthrough                              Material  Removed  parts)   parts)  (minutes)                                 ______________________________________                                        Vermiculite +                                                                 MgO                                                                           First Use SO.sub.2   3100   0       65                                        After first                                                                             SO.sub.2   3100   0       60                                        Reg'n                                                                         After 2nd SO.sub.2   3100   0       59                                        Reg'n                                                                         Perlite +                                                                     MgO                                                                           First Use NO.sub.2   930    0       15                                        After Reg'n                                                                             NO.sub.2   930    0       37                                        Vermiculite +                                                                 MgO                                                                           First Use HCl      45,900   0       33                                        After first                                                                             HCl      45,900   0       33                                        Reg'n                                                                         After 2nd HCl      45,900   0       32                                        Reg'n                                                                         ______________________________________                                    

EXAMPLE 4

A series of filtering runs was carried out wherein the performances ofinventive filters made with magnesium oxide, calcium oxide, copper oxideor calcined dolomite coated onto exfoliated vermiculite or expandedperlite were compared with the performances of filters of straightexfoliated vermiculite, expanded perlite, magnesium oxide and calciumoxide. Each filter bed was 2.25 inches in diameter and about 5 incheslong except the ones made with cupric oxide which were 1 inch indiameter and 3 inches long. The beds were exposed to the water-saturatedsimulated flue gas streams indicated in Table IV. The inventive filterswere conditioned by heating at 450°-550° C. for 15-30 minutes. Thevermiculite and perlite beds when used alone showed no ability to removesulfur dioxide. The straight magnesium oxide and calcium oxide bedsinitially removed sulfur dioxide, but rapidly developed a mud-likenature that stopped gas flow. All inventive filters captured sulfurdioxide effectively with no appreciable pressure changes across the bedduring runs. The results of these runs are summarized in Table IV.

                  TABLE IV                                                        ______________________________________                                                                    SO.sub.2                                                                      Conc.                                                               Feed      out from                                                                             Time,                                                        Gas       Filter,                                                                              min.,                                                Gas     SO.sub.2  ppm,   of 50%                                     Filter    Flow    Conc.     after  Break-                                     Material  L/min.  ppm       1 min. through                                    ______________________________________                                        Vermiculite                                                                             1        500      500     0                                         Perlite   1        500      500     0                                         MgO       1       1000      0      Clogged in                                                                    20                                         CaO       1       1000      0      Clogged in                                                                    15                                         MgO +     1       1000      0      3000                                       Vermiculite                                                                   MgO +     10      3200      1      420                                        Vermiculite                                                                   CaMgO.sub.2 +                                                                           1       1000      0      2300                                       Vermiculite                                                                   CaO +     1       3100      0      750                                        Vermiculite                                                                   CaO +     2       3100      0      590                                        Perlite                                                                       CuO +     1       3300      10     25                                         Vermiculite                                                                   CuO + Perlite                                                                           1       3300      150    22                                         ______________________________________                                    

Many modifications and variations of the invention will be apparent tothose skilled in the art in the light of the foregoing detaileddisclosure. Therefore, it is to be understood that, within the scope ofthe appended claims, the invention can be practiced otherwise than asshown and described.

I claim:
 1. In a process for removing sulfur oxides, nitrogen oxides,and hydrogen halides from a gas contaminated with same in which the gasis contacted at a temperature below 250° C. with particulate solidacceptor comprising a mineral support having a coating containingdivalent metal oxide, hydroxide, or both, the support beingwater-moistened particles selected from the group consisting ofexfoliated vermiculite, expanded perlite, or a mixture thereof, theimprovement which comprises:heating the acceptor before its first use ata temperature of at least about 400° C. until it is conditioned for theaccepting purpose; dampening the conditioned acceptor with water;contacting contaminated gas with the acceptor; and separating resultingcleansed gas from the acceptor.
 2. The process of claim 1 wherein thecoating comprises MgO and water.
 3. The process of claim 1 wherein thecoating comprises CaO and water.
 4. The process of claim 1 wherein thecontaminated gas is humidified before or during contact with theacceptor particles.